1. Field of the Invention
This invention relates to a printed wiring board having a rigid substrate as a core material, on at least one surface of which is layered a flexible substrate, and to a manufacturing method for the printed wiring board.
2. Description of Related Art
Among the printed wiring boards, known so far, there is a so-called rigid substrate, carrying a tough core material formed, e.g., of a glass epoxy resin, with the core material carrying a wiring pattern, and a so-called flexible substrate having a flexible core material carrying a wiring pattern.
Further, in portable high-performance electronic equipment, such as digital mobile equipment, in which a demand is raised for increasing the transmission rate and the memory capacity, there is also raised a demand for reducing the size and weight of the equipment in order to improve its portability. In keeping up with this demand, it is required of the printed wiring board to increase the number of I/O pins and to reduce the weight of the semiconductor.
Among the aforementioned printed wiring boards, there is a multi-layered printed wiring board having plural substrates layered together and plural electrically conductive layers.
However, in a multi-layered rigid substrate, a through-hole needs to be bored in each substrate for establishing electrical connection across the respective conductive layers. If this through-hole is formed, the conductive layer provided on the core material and the inner wall surface of the through-hole need to be plated. For this reason, in a multi-layered rigid substrate, the copper foils, forming the respective conductive layers, cannot be reduced in thickness, so that difficulties are encountered in further reducing the thickness and weight of the entire substrate or in forming fine patterns thereon. Moreover, in the multi-layered rigid substrate, a wiring pattern is formed on each surface of the core material, a further core material is bonded on each surface of the core material, now carrying the wiring pattern, and a wiring pattern is formed on the so-bonded core materials. Thus, if malfunctions occur in the course of the manufacturing process, the substrates bonded previously need to be discarded in their entirety, thus decreasing the yield and rendering it difficult to improve the production efficiency and to reduce the production cost.
It is therefore an object of the present invention to provide a printed wiring board that can be reduced in weight by reducing the size and the weight of the entire substrate and a method for producing this printed wiring board.
In one aspect, the present invention provides a printed wiring board, including a rigid substrate having a land on at least one surface of a core material and a flexible substrate including a bump, protuberantly formed on at least one surface of an insulating layer for establishing electrical connection to the land, and a land on its other surface in which the rigid substrate and the flexible substrate are molded as one with each other, with the interposition of an adhesive in-between, so that the land and the bump face each other.
In another aspect, the present invention provides a method for producing a printed wiring board, including the steps of forming a rigid,substrate, including a land on at least one surface of a core material, forming a flexible substrate, including a bump protuberantly formed on at least one surface of an insulating layer for establishing electrical connection to the land and a land on its other surface, and molding the rigid substrate and the flexible substrate together by vacuum hot pressuring, with the interposition of an adhesive in-between, so that the land and the bump face each other.
In the printed wiring board, according to the present invention, in which a flexible substrate is layered on at least one surface of a rigid substrate exhibiting toughness, it is possible to provide more circuits than is possible with a layered rigid substrate, thus enabling the entire device to be reduced in size. The printed wiring board is produced by bonding a flexible substrate to the rigid substrate exhibiting toughness, so that bonding may be easier than if flexible substrates are bonded together. Moreover, with the present printed wiring board, obtained on bonding the rigid substrate and the flexible substrate, carrying completed respective wiring patterns, electrical connection can be made to any desired layer in contradistinction from the case of a multi-layered rigid substrate where there are provided plural conductive layers.
In addition, in the method for producing the printed wiring board, according to the present invention, the technique of forming wiring patterns on both surfaces of the core material, bonding further core materials on both sides of the first-stated core material, now carrying the wiring patterns, and forming further wiring patterns on the second-stated core materials, as in the case of the conventional method for producing a multi-layered rigid substrate, is not used, each substrate which is to make up the printed wiring board may be bonded in position after inspection, thus improving the production yield. Moreover, since the number of plating operations for the copper foil for the flexible substrates is less than that for the rigid substrate, the flexible substrates may be of lesser thickness than the rigid substrate. Since the printed wiring board is produced by layering these flexible substrates, it can be of a lesser thickness than the layered rigid substrate.